Synopsis: Universal Pairing Symmetry

Superconductivity in the CeMIn5 family, where M can be cobalt (Co), rhodium (Rh), or iridium (Ir), occurs in close proximity to a phase with antiferromagnetic order. In these materials, charge carriers have a large effective mass (hence the name “heavy fermion” superconductivity), which implies that conventional attraction via phonons cannot be responsible for the pairing that gives rise to superconductivity.

One hint to the pairing mechanism can come from studying the symmetry of the superconducting gap. CeCoIn5 and CeRhIn5 are widely accepted to have nodes in the superconducting gap (dx2-y2 symmetry), suggesting the pairing interaction is driven by antiferromagnetic spin fluctuations. But for the third member of the family, CeIrIn5, the superconducting gap symmetry has remained controversial because of discrepancies between different measurements.

In a Rapid Communication appearing in Physical Review B, Shunichiro Kittaka from the University of Tokyo, Japan, and collaborators report experiments that resolve this controversy and show that the gap in CeIrIn5 also has dx2-y2 symmetry. They measured the specific heat of a sample of CeIrIn5 at multiple orientations with respect to an external magnetic field down to temperatures as low as 80 millikelvin. Kittaka et al. find that the field-angle-resolved specific heat measurements show a fourfold angular oscillation that is consistent with theoretical calculations that assume CeIrIn5 has a gap with dx2-y2 symmetry. These results suggest a universal pairing mechanism for the CeMIn5 family of heavy fermion superconductors, an important clue to developing a complete theoretical understanding. – Sarma Kancharla